Paper is manufactured as a continuously formed web on a papermaking machine. In the last 30 years the speed at which paper is manufactured has been substantially increased. The speed at which the paper web is formed has doubled from approximately 3,000 feet per minute to upwards of 6,000 feet per minute on today's state of the art machines. The manufacture of paper starts with wood fibers suspended in is water to form a very dilute solution composed of over 99 percent water. The fiber suspension is directed onto a forming wire or between two wire screens in the forming section of a papermaking machine.
After the paper web has been formed it is pressed to approximately 50 percent water content in the pressing section of the papermaking machine. The pressing section is followed by tiers of dryers which typically employ steam heating to dry the web until it contains only about 5 percent moisture. The dried web is then smoothed by passage through a calender. And the web is then wound into reels of paper at the dry end of the papermaking machine for further processing or sale.
The search for ways of improving the rate of the production of paper has also lead to increases in the width of the paper web being formed to approximately 400 inches. Increasing the width of the paper web being formed beyond 400 inches does not, at this time, appear to be practical because of the difficulty of controlling the shape of the rolls and the pressure between rolls used in the papermaking machine.
Thus, the future improvements in papermaking are seen in the reengineering of each component of the papermaking machine to increase the speed of the machines and improve the operating efficiencies. Further, efforts are needed to decrease the number of individual rolls employed in such a way as to shorten the overall length of the papermaking machine, with the goal of reducing the overall size of the machine. At the same time the length of the papermaking machine is being reduced the individual components--from the former through the winder--are being engineered to function better at high speeds: The entire papermaking machine is being engineered for better and automatic threading; the paper is being supported throughout its travel through the papermaking machine; and active feedback and monitoring of the paper web's properties are being employed. These improvements should set the stage for further increases in paper forming speeds. One section of the papermaking machine which has received considerable attention is the dryer section. A dryer section in a typical papermaking machine takes up substantially more than half of the overall length of the machine and considerably more than half of the energy used in forming the paper.
Some of the problems with conventional two-felted, two-tiered dryer sections where the wet web passes from one cylinder to the next in a generally serpentine fashion, are: the existence of long unsupported "open draws," problems with tail threading, sheet flutter in the open draws, cross-directional sheet shrinkage, and inefficient ventilation of evaporated water.
Some of the problems, including sheet flutter, sheet shrinkage and vapor ventilation, have been solved by replacing the two-felted, two-tiered dryer sections with single-tier BelRun.TM. dryer sections as manufactured by Beloit Corporation of Beloit, Wis. Extension of the single-tier concept to include more dryer cylinders in the single-tier configuration has provided significant improvements in the operation of the dryer section.
The use of both top-felted single-tier dryer sections followed by bottom-felted single-tier dryer sections has improved sheet one-sidedness. Such systems are exemplified by the Beloit Bel-Champ.TM. dryer section. One advantage of the single-tier dryer section is its ability to have the tail threaded through the dryer section without the use of threading ropes. A further advantage is the elimination of open draws where sheet flutter can result in wrinkled paper or even paper breaks. Still further advantages of the single tier dryers include better access for removing broke, improved ventilation, reduced web shrinkage, improved sheet surface and strength properties, and improved machine runability. Many of these advantages are achieved through the application of vacuum to the intermediate vacuum rollers. Open draws between dryer sections are eliminated through the use of transfers where the web is supported by two felts as it transfers between dryer sections.
Constraining the sheet while it moves through the dryer section increases sheet restraint in the cross-machine direction, which reduces shrinkage in the cross-machine direction. In a conventional dryer section, the web is constrained only approximately 58 percent of the time as it moves through the dryer section of the papermaking machine. By comparison, fabric pressure and vacuum rolls hold the paper web in the BelRun.TM. approximately 84.3 percent of the time it is in the drying section.
Although the BelRun.TM. and Bel-Champ.TM. dryer configurations offer significant improvements over other dryer section designs, one of the limitations of the BelRun.TM. and Bel-Champ.TM. single-tier dryer sections is that the cross-directional sheet restraint is not applied for 100 percent of the cycle time. A further limitation is the large number of vacuum rolls required to restrain the wet web from cylinder to cylinder in between sections.
What is needed is a dryer section with improved web constraint in the cross-machine direction and greater compactness in overall length of the dryer section.